Case 011
The Swapped Envelope
A sealed envelope was replaced with an empty one.
Rule: exactly one suspect is guilty. The guilty suspect lies. Every innocent suspect tells the truth.
Parker
“Either Harper or Alex is guilty.”
Blake
“Neither Harper nor Alex is guilty.”
Harper
“Blake and Finley are both innocent.”
Alex
“Harper is innocent.”
Finley
“Neither Blake nor Alex is guilty.”
Who is guilty?
Solution: Parker
If Parker is guilty, Parker's statement is false and every other statement is true. Testing the other suspects creates at least one contradiction.